1. Field of the Invention
The invention relates to a method for coating high-temperature components by means of plasma spraying, in particular gas turbine components. The invention also relates to a coating device having an infrared camera.
2. Related Art
In addition to other thermal coating methods, because of its flexible use options and a good economic balance, plasma spraying is of great importance in the production of coatings for protecting components, for example against corrosion by hot gases. Vacuum plasma spraying (VPS), low-pressure plasma spraying (LPPS) and atmospheric plasma spraying, inter alia, are among the various known methods.
In plasma spraying technology, a coating is produced by directing a very hot plasma jet onto the substrate to be coated while feeding material which is to be applied. The coating material is present in this case mostly as powder or wire and is fused during transport by the plasma jet before striking the substrate. It is therefore possible in principle to produce the most varied layer thicknesses using very different coating materials and substrate materials. It is possible to use metal powder and ceramic powder in the most varied mixtures and grain sizes as long as the starting material has a defined melting point. An MCrAlY layer, M standing for the metals Ni and Co, is used, for example, to coat gas turbine buckets with a layer protecting against corrosion by hot gases.
The type and quality of the layer is influenced, inter alia, by the pore content, the oxide and nitride content and by its adhesive properties. In addition to the roughness of the surface, the mutual diffusion of the different materials or chemical reactions are important adhesion mechanisms. It is frequently necessary to apply an adhesion promoter layer before applying the actual protection layer, in particular whenever there is a need to balance different coefficients of thermal expansion.
Various methods are applied to monitor the quality of the coating. Preference is to be given in this case to nondestructive tests such as are provided by ultrasonic or infrared technology, for example. In the case of the first-named methods, it is frequently disadvantageous that the inspection instruments touch the surface of the workpiece, thereby limiting the use options, for example to specific component geometries. Furthermore, errors frequently occur owing to surface contamination and surface irregularities or other surface anomalies. The inspection of the component consists in observation over a large area and in an averaging fashion.
Many of these disadvantages are eliminated in the case of infrared technologies. They are based on the fact that, in a fashion correlated with the temperature of the component, each material absorbs and emits electromagnetic radiation which is recorded by infrared detectors. The infrared methods can be used quickly and flexibly and can be applied without difficulty with controlling and regulating systems.
An infrared thermography method represented in U.S. Pat. No. 5,111,048 can be used to detect cracks which arise, for example, due to stresses in the layers. In this case, laser radiation is used to produce contrast between the fault positions and the remainder of the surface. By contrast with the undisturbed surface,fault positions exhibit other absorption or emission properties of electromagnetic radiations. It is disadvantageous, inter alia, that this method cannot be used in a coating chamber during coating, and that the radiation must firstly be excited by external radiation means independently of the heating.
A device and a method for inspecting the thickness and the faults of the coating by means of an infrared technique is described in GB 2 220 065. In this case, the coated component is irradiated by a short infrared pulse and the response beam is recorded by an infrared camera. The region to be inspected is illuminated in this case more homogeneously than in the method described above. It is disadvantageous, inter alia, that at higher process temperatures the infrared radiation of the heated component and of the flash lamp overlap in a way which is difficult to separate for the purpose of detection and evaluation provided in the measurement method.
The monitoring methods set forth above and others, as well, are generally carried out after fabrication of the coating. However, it is desirable to carry out online monitoring as early as during the coating, in order to intervene for control purposes, if required, and/or to control the method with the aid of the results. Moreover, monitoring and control, associated therewith, of the method parameters is indicated during the process in order to ensure the quality and to improve the method.
A method for online monitoring of the coating during the coating operation is described in U.S. Pat. No. 5,047,612. An infrared detector is used to determine the position of the jet spot of the plasma jet on the component to be coated, and the application of the coating is influenced during the coating by controlling the powder flow and the carrier gas of the powder. It is disadvantageous in this case that the setting of process parameters is performed essentially independently for each component. The control of the powder distribution does not, moreover, constitute per se a sufficient condition for a reliable adhesion of the coating which satisfies the operating requirements.
By contrast, the surface temperature of the component to be coated is of fundamental importance for forming the various protective functions of the coating. The abovementioned MCrAlY layers achieve their protective function by, for example, forming aluminum oxide or chromium oxide layers. Attack by oxidation, in particular, is thereby prevented in the base material. The oxide layers are formed differently depending on the surface temperature of the component. In accordance with recent results, the surface temperature of the substrate and the temperature gradient on the component surface are likewise to be accorded greater importance for the adhesion of different metal/ceramic layers in the plasma spraying process (see, for example, Proc. Int. Therm. Spr. Conf. 1998, Nice, France, pages 1555 ff.).
Pyrometers are frequently used at a point on the surface of the component which is to be freely defined for the purpose of temperature measurement during plasma spraying. However, these supply only point measurements, and in the event of a movement of the bucket during the conduct of the process there is a risk that pyrometric temperature measurement will be carried out at differing locations on the bucket surface. The temperature measured in this way is therefore subject to large fluctuations which cannot be calculated.
It is therefore the object of the present invention to improve the initially mentioned method/the initially mentioned device such that the quality of the layers produced can be observed and set reliably and reproducibly during the coating method
An area-wide overview of the component surface is obtained in real time by means of measuring the thermal distribution of a surface region of the component with the aid of an infrared camera for the purpose of the present invention. Measurement of the thermal radiation with the aid of an infrared camera has certainly already been used to monitor the application of powder during plasma coating, for example in the abovenamed known method according to U.S. Pat. No. 5,047,612. By contrast, in the present invention the exact absolute temperature distribution of the overall component surface or of selected, predetermined sections of the component surface is determined exactly and as a function of time. An infrared camera according to the invention corresponds to an infrared-sensitive CCD array with optical systems for imaging the component on the CCD array, and to intensity- or frequency-dependent evaluation devices. The temperature distribution is determined from the thermal distribution by comparing the thermal radiation of the component surface measured using the infrared camera with a radiation reference means. Setting the thermal distribution and/or the temperature distribution determined therefrom with the aid of an adjustable method parameter in a fashion associated with the measurement of the thermal distribution or the temperature distribution is essential to the present invention. By setting the method parameter, the surface temperature is corrected with regard to its absolute magnitude for the purpose of reaching a threshold temperature.
The radiation reference means is brought by a heater to a temperature which can be set if required and is determined exactly by a temperature monitoring element. The thermal images of the radiation reference means taken with the camera can be assigned absolute temperature values in a simple way such as, for example by means of color comparisons or, for example in the case of an upstream radiation filter, by intensity comparisons, and these absolute temperature values can be transferred onto the thermal image of the component. The surface temperature of the component is then adjusted by setting the method parameter, and is brought reproducibly and accurately into a range which is advantageous for the formation and adhesion of layers, while taking account of the special properties of the surface region respectively present. An essential condition for good adhesion is then achieved when a threshold temperature is exceeded.
In general, color comparisons can be undertaken xe2x80x9cby eyexe2x80x9d with a high sensitivity. For example, setting a predetermined temperature of the radiation reference means close to the threshold temperature which is to be set results in a simple criterion, which can be monitored quickly and reliably, for exceeding or falling below the threshold temperature simply by visual comparison of the thermal radiation shots of the component and of the radiation reference element. However, it is also possible to make sensible use of evaluation by means of EDV, for example electronic comparison of color value or intensity.
The method of this invention provides reproducible results and ensures as early as during the coating operation that the adhesive properties of the layer to be applied are monitored exactly and in a way which can be handled variably. For reasons of clarity, the temperatures can even be set by hand while maintaining accuracy and reproducibility. The high spatial accuracy or a very good resolution has a favorable effect, in particular in the case of complex surface regions which are to be coated.
When producing relatively large batch-quantities of coatings for components, it is possible, by setting a tested method parameter, to achieve with simple steps an increase in the reproducibility of the coating results, an improvement in the reliability of the coating, and a constant high quality. This can also be carried out for quality assurance within the framework of quality management of such a process control. The proposed method is therefore well suited to the industrial production of coatings for high-temperature components.
It is advantageous, furthermore, to use the method parameter to set, in the surface region of the component, a temperature distribution for which predetermined temperature differences and/or temperature gradients are not exceeded. Inhomogeneities in the temperature distribution, in particular strong local fluctuations, that is to say large temperature gradients, can lead, despite a generally very high average temperature, to reduced adhesion of the coating. Temperature gradients can arise, for example, from uneven heating or varying component properties such as, for example, different thicknesses of the material. In addition to setting the parameter for the purpose of reaching a threshold temperature, it is possible by setting the parameter to limit temperature fluctuations of the surface by maintaining maximum temperature differences, and to set a uniform temperature distribution.
Furthermore, detecting the thermal radiation by means of an infrared camera can show temporal fluctuations in the temperature distribution, which result from power fluctuations in the heating source, for example, specifically in an in-situ fashion and with maximum temporal resolution, for example 10-50 images/sec. The parameter is advantageously set in this case on the basis of empirical values or measured values and by coordination with the measured, time-dependent temperature distribution.
The threshold temperature is advantageously set with regard to an optimum adhesive power of the coating on the component, and/or the temperature differences and/or temperature gradients are permitted for the same purpose only within predetermined limits. Different materials, in particular material combinations of layer material and substrate material, render it necessary when setting the temperature distribution of the surface regions of the components to achieve different threshold temperatures, and this is possible by varying the setting of the method parameter.
It is possible with the aid of the present invention to achieve a flexible, quick and accurate setting of the threshold temperature as required by setting the parameter as a function of the measured temperature distribution. In addition, there is a possibility of thereby adjusting to different component properties. By controlling the method parameter, it is possible to react individually to the temperature fluctuations, and limits of temperature differences required for the adhesion of the coating to be observed.
It is possible, furthermore, to use component-specific and material-specific parameters in the case of process monitoring and process control by hand or by means of EDV support. The influence of different material strengths, for example owing to the variations in the thermal conductivity of the components, can thereby also be taken into account. In applying multiple, and also different coatings to a component, the threshold temperatures, and thus the coating temperatures, can be adapted quickly and individually by means of stored, material-specific magnitudes of the method parameters.
It is proposed to set a predetermined threshold temperature in each case at a plurality of regions on the surface of the component. Preferably precisely at points on the component subject to particular loads in later use, for example parts of gas turbines subject to the hottest and strongest flows and mechanical loads, to ensure optimum adhesion, thus ensuring functionality. It is always possible by means of the present invention for these requirements to be fulfilled as necessary. A jet used to heat the component can be guided in accordance with the requirements over specific points on the component which cool more quickly. Simultaneous monitoring is provided virtually at any instant by observation and control with the aid of the infrared camera.
It is advantageous when the method parameter is controlled by comparing the temperature distribution of the surface region of the component with a desired temperature distribution. When certain temperature distributions have proved to be particularly advantageous in test measurements, trial runs and also during the actual coating, it is desirable to be able to use this temperature distribution for following coatings. Thus, if a constant temperature distribution with temperatures higher than a threshold temperature has proved to be sensible; the temperature distribution is then set for the entire surface in accordance with this constant temperature. This can be carried out quickly by hand. By using magnitudes of the process parameter stored in a control loop and checked, a temperature distribution can, moreover, be set after comparison with the temperature distribution of the component surface supplied by the infrared camera.
The component is advantageously preheated and/or heated during plasma spraying with a plasma jet, and a parameter of the plasma jet is set as the method parameter. The adhesion of the layer on the base material is positively influenced by a high preheating temperature. The preheating temperature is important for good adhesion not only of the first, but also of all subsequent layers applied in turn thereto, since these later layers can only adhere as well as the first ones. A temperature comparable to the preheating temperature should also be maintained during the plasma spraying, and is advantageously to be achieved by heating with the plasma jet. By comparison with inductive resistance heating, for example, heating with the plasma jet essentially ensures that the outer layers important for the coating are heated. The component material, which possibly cannot withstand the high temperatures over a lengthy time, is damaged only minimally. At the same time, the surface can be cleaned with the plasma jet, by polarization of the component, explained in more detail below, which also improves the adhesion. However, it is possible in this case, that stronger gradients are set up in the temperature distribution and counteract good adhesion. It is therefore advantageous when preheating the component to have the entire component viewed by the infrared camera, and to be able to control the method parameters accordingly.
Moreover, the two operations of heating and coating, which frequently overlap one another in an uncontrollable way during the plasma spraying process, can be monitored and controlled separately from one another by means of the method presented. The power of the plasma jet can be controlled as required by setting its method parameters. This permits a quick reaction to the results obtained by the infrared camera as regards the temperature distribution. Given the same travel path or the same scanning method of the beam on the, component surface, good reproducibility of the method can be ensured by storing and evaluating the data for the plasma jet. This ensures a better quality of the layers, and increased productivity.
In particular, the current of a radiation source of the plasma jet can be set as the method parameter. This variable can be controlled inexpensively and permits precise coordination of the energy input of the plasma jet into the surface of the component as required by the predetermined temperature distribution.
In the present method, the position of the component relative to the plasma jet can be varied, and the temperature distribution of the surface region of the component can be determined in different relative positions with respect to the plasma jet. It is possible in this way to undertake individual monitoring of the various surface regions of the component without needing to remove the component. The various component positions can be stored. This permits the component position to be assigned reproducibly to a magnitude of the method parameter. For applying the method for further components of the same type, it is sensible in this case to use stored data, for example the starting point or assignment of the component position, for the purpose of controlling the method parameter for each component of the series.
During plasma spraying, the component can be rotated with an optimum alignment of the rotation axis of the component relative to the infrared camera. Thus, the entire surface of the component can be coated completely and uniformly, and monitoring of the surface temperature distribution can be undertaken simultaneously by means of the infrared camera without altering the setting of the plasma jet. This monitoring function can be undertaken in the form of short-term measurements, that is to say separately for each surface region, taking account of the rate of rotation. The spatial resolution should be very precise in this case. In order to achieve the threshold temperature, it is possible to set the method parameters in a fashion adapted to the surface conditions.
Alternately, long-term measurements can be taken, that is to say measurements over times which vary in the range of several rotational periods. The result of these measurements are then average temperature values averaged over the time and the circumference of the rotating component in the direction of rotation. This type of measurement is quick and can be done inexpensively. The results can then be compared in turn with the threshold temperature.
The present plasma spraying device preferably comprises a holder for continuous rotation of the component about its longitudinal axis. This type of rotation can be carried out in a stable fashion and ensures the greatest possible effectiveness with regard to the coating rate and a uniform layer application. In order to ensure, simultaneously with good layer application, optimum measurement of the temperature distribution of the component surface as well, special conditions are advantageously set for the angular ratio of the rotation axis to the plasma jet and camera alignment. In particular, in this case one should avoid having the solid angle in which the plasma radiation is reflected intersect with the visual angle of the infrared camera. If not avoided, this setting would swamp out the entire shot as a result of camera receiving the direct and/or reflected radiation of the plasma jet. The infrared camera is therefore arranged outside the solid angle of reflection of the plasma jet.
The temperature distribution of the surface region of the component is advantageously determined as a function of time, and the method parameter is set in accordance with the temporal response of the temperature distribution. The infrared camera permits the entire temperature distribution to be recorded in one step. With regard to continuous monitoring of the development of layer quality, it is advantageous to detect the temperature distribution as a function of time, in order to determine the material response and the jet response, and to be able to set a corresponding, time-dependent function of the method parameter.
The positional variations of the component relative to the plasma jet, on the one hand, and a method parameter of the plasma spraying, on the other hand, can be coordinated with one another in accordance with the temperature distribution such that temperature gradients on the surface of the component are reduced. For example, the method parameter can be set such that less energy is transmitted per element of area. This can be done, for example, by moving the plasma jet more quickly relative to the component surface. The energy transmission per time unit remains the same, but is more uniformly distributed. This reduces the temperature gradient. On the other hand, too low an energy transmission can also cause the surface temperature to drop too sharply. The power of the plasma jet can then be raised. In order to achieve a high-quality surface layer, it is necessary to coordinate the various positions of the component precisely with the changes in the parameter in accordance with the determined temperature distribution.
When short-term shots are carried out during component rotation, it is advantageous to trigger successively occurring shots taken with the infrared camera as a function of the rotational period of the component. By shooting the same component regions in different states, it is possible to undertake precise measurement of the temporal temperature response of the surface temperatures, and to adjust the method parameter with the aid of the results. It would otherwise be impossible to exclude sources of error when determining and controlling the temperature, owing to the displacement of the surface region considered.
The triggering is carried out with a temporal spacing of a quarter of the rotational period or an integral multiple thereof. It is ensured in this way that either the front side or the rear side of the component, or the sides of the component, are inspected. The two sides can, for example in the case of a turbine bucket, have different forms and material thicknesses of the component, and therefore store the input energy of the plasma jet at different intensities. Consequently, different forms of temperature gradients are present, and this may require adjustment of the method parameter of the plasma jet.
It is proposed that the radiation reference means can be heated independently of the heater for plasma spraying. This permits the material of the radiation reference means to be heated completely and, in particular, uniformly, for example by inductive heating or direct heating, for example resistance heating. This supplies an important precondition for the correct surface-independent comparison of the temperatures of the reference means and the component to be coated.
Furthermore, the temperature of the radiation reference means is advantageously to be measured with the aid of a thermocouple. Determining the temperature with the aid of a thermocouple yields measured values which are independent of surface properties. After calibration, measurement with the aid of the thermal couple, or else another independent temperature-measuring element supplies reliable values of the absolute temperature which can be used for a comparison with the results of the thermal radiation measurements of the component by means of the infrared camera.
It is proposed that the radiation reference means is arranged in the measuring field of the camera inside the chamber next to the component to be coated. This permits the infrared camera to detect simultaneously the radiation reference means and the component to be coated. This can be particularly advantageous in the case of rapidly varying radiation conditions and reflections which can influence the measurement results. Detection in the same measuring field permits measurement under the same environmental conditions, and this is advantageous, in particular, with rotated or otherwise displaced components, because of the quickly changing visible surfaces. The environmental conditions are also substantially influenced by pollution by coating material on the observation window or by the infrared components in the radiation of the plasma jet. It is therefore particularly advantageous for the purpose of ensuring unfalsified measurement results to fit the radiation reference means inside the coating chamber.
The camera is arranged and designed such that it can be used to detect at least the entire surface, facing it, of a turbine bucket. Particularly when large temperature gradients are to be expected because of great differences in the component properties, for example in the component material thickness, it is advantageous to be able to cover the entire surface. The particular arrangement of the camera of the present invention permits this to be done without any problem. Particularly advantageous in this case is the detection, which is easy to carry out, and control of the temperature distributions of edge regions and regions of small radius of curvature such as occur in the case of turbine buckets in the region of the bucket ends. This is important because additional strong mechanical and thermal loads act there on the coating during use by comparison with flat surface regions.
The infrared camera is fitted at one end of an outwardly projecting stub of the coating chamber. A glass window is fitted at the end of the stub and permits a view into the coating chamber, which is provided with a seal for ensuring an effective vacuum and is thereby subject only to low pollution from process dust. The proposed device reduces the frequency at which the apparatus needs to be maintained and cleaned. It is favorable for the infrared camera shots when the stub has a conical shape with a wide, free angular aperture range. This shape is then adapted to the visual range of the infrared camera and permits optimum shots of the component.
The glass window advantageously consists of a special glass having a transmission for wavelengths between 2-5 xcexcm which is adapted to the measuring range of the camera. This measuring range corresponds to that infrared radiation region in which a large fraction of the radiation of the component surface is emitted. This region of radiation is sufficiently well distinguishable from the mutually overlapping, wideband infrared fraction of the plasma jet. The wavelength region of 2-5 xcexcm inspected is far removed from the maximum wavelength of the temperature radiation of the plasma jet and, by comparison with the other radiation regions of the plasma jet, is of lower intensity. In the case of the present online monitoring of the coating, in particular, this is important in order to obtain an accurate, well resolved and clear image of the temperature distribution of the surface of the component.
The glass window advantageously consists of sapphire glass. This type of glass, which contains Al2O3, has optimum transmission properties in the desired region. The glass is commercially available and can be adapted in functional terms to the device according to the invention.